Inferior rectus muscle

The inferior rectus is one of the four recti muscles of the eye. It lies in the inferior part of the orbit, directly beneath the eyeball, and is responsible for coordinated movements of the globe.

Origin and Insertion

• Origin: Common tendinous ring (annulus of Zinn) at the apex of the orbit.

• Insertion: Inferior aspect of the sclera, anterior to the equator of the eyeball, approximately 6–7 mm posterior to the corneal limbus.

Note: Like all recti muscles, the inferior rectus inserts into the anterior half of the globe, whereas the oblique muscles insert posterior to the equator.

Nerve Supply

• Oculomotor nerve (cranial nerve III) — inferior division.

Actions

• Primary action: Depression of the eyeball.

• Secondary actions: Adduction and lateral rotation (extorsion).

Functional Note

The inferior rectus runs forward and slightly outward from the back of the orbit, making an angle of approximately 23° with the visual axis when the eye looks straight ahead. Because of this outward alignment, its line of pull is oblique, not purely vertical.

When the muscle contracts:

• The downward component of its pull depresses the eye.

• The inward (medial) component slightly adducts the eyeball.

• Its attachment to the lower surface of the globe causes the inferior pole of the eye to move medially, producing lateral rotation (extorsion).

In simple terms, the inferior rectus muscle pulls the eye down and slightly in, causing depression, adduction, and extorsion simultaneously.

When the eye is abducted about 23°, the visual axis aligns with the long axis of the muscle, so it acts mainly as a depressor.

Clinical Correlation

To test the inferior rectus, the examiner asks the patient to first look laterally, then downward. This aligns the visual axis with the muscle’s long axis, isolating and testing its pure depressing action.